(a) TGF-β–producing cells, but not Th1 cells, inhibit Th2 cell–induced airway hyperreactivity in SCID mice. SCID mice received OVA-specific Th1, Th2, or TGF-β–producing cells (2.5 × 10⁶ cells/mouse) intravenously plus intranasal OVA (50 μg) 18 hours before cell transfer. Other SCID mice received a mixture of OVA-specific Th1 and Th2 cells (2.5 × 10⁶ cells/mouse) or a mixture of OVA-specific TGF-β–producing cells and Th2 cells (2.5 × 10⁶ cells/mouse). To examine the specificity of the effect, some mice received KLH-specific TGF-β–producing cells and OVA-specific Th2 cells. Control mice received either OVA only (no cells transferred) or cells only without antigen (data not shown). One and 2 days after the adoptive cell transfer, OVA was again administered intranasally. Three days after adoptive cell transfer, airway hyperreactivity in response to increasing concentrations of inhaled methacholine was measured in a whole-body plethysmograph. Data are expressed as mean percent above baseline (± SEM, n ≥ 6 for each data point). Cell transfer without intranasal administration of OVA resulted in minimal airway hyperreactivity (data not shown). (b) Anti–TGF-β mAb abolishes the inhibitory effect of TGF-β–producing cells on Th2 cell–induced airway hyperreactivity. Neutralizing mAb 2G7 (500 μg/mouse) specific for active TGF-β was given intraperitoneally to SCID mice that received either a mixture of TGF-β–producing cells and Th2 cells (2.5 × 10⁶ cells/mouse) intravenously or Th2 cells alone intravenously plus intranasal OVA on the day of cell transfer. Results are provided as mean percent above baseline (± SEM, n ≥ 5 for each data point).